Selecting Seals for the Uptick in Upstream Oil Production

Over the years, upstream oil operations have improved their efficiency and capability of pulling greater quantities of oil out of the ground. Engineers are achieving this by drilling deeper, using enhanced oil recovery (EOR) and running at higher temperatures than ever before. Although effective, these tactics can pose challenges to the seals used in pumps and compressors, as they often create demanding environments for the seals to operate in.

More Than Just Oil
The oil content found in wells varies and can depend on the location, climate and elevation—as well as other factors. A common variation in crude oil is whether it is considered heavy or light. Light crude oil contains mostly hydrocarbons. As impurities in the oil increase, it becomes heavier, thicker and more viscous and is referred to as heavy crude oil. Impurities can include heavy metals, aromatic compounds (chemicals like toluene, xylene and benzene) and resins, making the heavy crude oil less valuable and more time-consuming to process. Tar sands, bitumen and asphalt are all considered heavy. The heavier the crude oil is, the more challenging it is for the seals in the processing operation, as abrasives and sticky resins tear and destroy seals, while aromatic compounds can cause potential chemical attacks.

From well to well, the amount of sulfur content can also vary. Oil with low amounts of sulfur is called sweet crude oil. Sour crude oil, in contrast, has high amounts of sulfur, which can be dangerous. The greater the sulfur content, the higher the probability of the presence of hydrogen sulfide (H2S), which is a poisonous, corrosive and flammable gas. Both sulfur and H2S have a signature rotten egg smell, which makes them easily identifiable.

However, H2S is dangerous and can be deadly at higher concentrations. Even in small amounts, H2S can chemically attack seals—while polytetrafluoroethylene (PTFE) is fully compatible, nitrile rubber (NBR) is typically compatible in concentrations less than 10 parts per million (ppm), fluoro-elastomer (FKM) less than 2,000 ppm and hydrogenated nitrile butadiene rubber (HNBR) less than 50,000 ppm (5%).

Enhanced Oil Recovery
In many oil wells, only about 10% of the oil is simply pumped out of the ground. Enhanced oil recovery (EOR) is typically used next, which is where a separate line is drilled, and steam, water, carbon dioxide (CO2) or other media is injected behind the oil to loosen it up and make it easier to remove. The type of media injected into the oil well can have a significant impact on what the optimal seal material should contain, so it is important to understand the effects of each.

Water injection is the most popular EOR method. While the water used for injection can come from outside sources, produced water is often used (which is excess water pumped out of a different well) and can contain oil, gas, H2S and abrasive particles—such as rust, sand, salts, asphaltenes and waxes. These particles can shorten the lifespan of a seal. In addition, water is considered nonlubricating, so seals that work in oil may stick and tear when used with high amounts of water.

Steam injection, another EOR method, is used for heavy crude that is thick and viscous. The steam injected produces wet heat that loosens up the heavy crude oil. With steam, there are additional considerations of having higher temperatures, plus chemical resistance concerns of steam compatibility with the seal material.

With EOR, carbon dioxide can also be injected behind the oil to push it out. Oil wells often contain groundwater, and when carbon dioxide contacts water it can form carbonic acid, so it is important to confirm chemical compatibility. A larger issue with carbon dioxide is explosive decompression, which is when pressurized gases permeate rubber seals. If a rapid reduction of pressure occurs, the gas inside the rubber can expand violently, creating a fissure resulting in immediate leakage. For carbon dioxide or any gas EOR, it is important to choose seals that have excellent permeation resistance, or minimal porosity, to prevent gas from entering the seal.

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